The fact that sealed pressure joints must, on occasion, perform under conditions of high temperature and pressure is well known in the art of fluid and material transfer. One such industry in which such connections are frequently used is in the oil production industry for the transfer and control of highly flammable liquids and gases. Seals are found in flanged pipe connections and at the interface between the valve body and valve bonnet in a control or other type of valve. In this industry it is well known that such flat-faced surface seals must undergo conditions of extreme temperature, corrosive environment, thermal shock, pressure cycling, vibration, or any combination of these or other conditions. One effective sealing means in use today is a metal-to-metal seal formed by the insertion of a seal ring between the flat-faced surfaces for the formation of a metal-to-metal seal therebetween.
Recent design specifications for well head equipment have required that the seals used in this equipment be able to resist extremely high temperatures such as would be present in the event of a well head or other fire. The abundance of flammable material present in such applications, as well as the potential for increased harm if a leak should occur, combine to make this requirement an essential and desirable goal. This requirement has been made even more demanding by the imposition of an additional condition that the seal employed for fire resistant well head service be able to maintain its integrity under conditions of uneven heating such as might occur if an intense flame was in contact with one portion of the sealed equipment while the other portion was subjected to a relatively cool environment.
The uneven heating requirement of this new standard has resulted in less than satisfactory test performance by the seal designs currently in use. The uneven expansion of the pressure containing members can allow leakage past the seal resulting in failure of the equipment and seal to meet the exacting standards of the fire resistant test. Such a test may be exemplified by the requirements embodied in various industry association's standards, those of the American Petroleum Institute being typical.
What is required is a metal-to-metal sealing system which is effective under conditions of extreme pressure and temperature, and which is suitable for use in well head applications wherein test requirements call for maintenance of seal integrity under conditions of extremely uneven heating of the subject equipment.